Wood composites, such as doorskins, are molded articles that are adhesively secured to a wood frame to provide the decorative and water-resistant outer surfaces of a door. Such wood composites traditionally are formed by the high temperature pressing of wood fragments in the presence of a binder.
Conventionally, the binder utilized in the high temperature pressing of the wood composite is a thermosetting resin, such as a phenol-formaldehyde resin, or the like. The traditional thermosetting resins require pressing temperatures in the range of about 350.degree. F. to about 450.degree. F. to cure the resin. Other resins, such as a melamine-formaldehyde resin or a urea-formaldehyde resin, do not require a high temperature cure, but also do not provide the water resistance properties of a resin cured at a high temperature.
Traditionally, the need to cure such a resin made it necessary to dry the wood fragments included in the wood composite to a moisture level of less than about 5%, and often to about 2%, by weight of the wood fragments, prior to pressing the wood fragments in the presence of a traditional resin binder. At wood fragment moisture levels of about 5% by weight or greater, the water interfered with the curing process of the traditional resins. In addition, when utilizing a traditional resin binder and dried wood fragments including less than 5% by weight moisture, a rehumidification step was required after pressing in order to increase the moisture content of the dried wood fragments and to provide a wood composite having acceptable commercial properties.
In general, the high temperature pressing of a traditional phenol-formaldehyde binder provides a surface that is porous with respect to the adhesive, but having a weak surface strength. In contrast, the high temperature pressing of an isocyanate binder provides a strong surface that is nonporous with respect to the adhesive. Therefore, molded wood composites including a traditional phenol-formaldehyde, or similar, resin binder exhibited the advantage of good porosity with respect to the adhesive used to bind the wood composite to a substrate, such as a wood frame. A wood composite exhibiting a good porosity with respect to the adhesive adheres more strongly to the underlying wood frame. Insufficient penetration of the adhesive into the wood composite decreases the adhesion between the wood composite and the frame.
However, the traditional phenol-formaldehyde binders provided weak surfaces that had to be reinforced by an additional post-press resin treatment. The problem of weak surfaces that had to be reinforced without introducing a poor porosity with respect to the adhesive, and the resulting poor adhesion to an underlying wood frame, is a problem whose solution has evaded investigators in the art. Surprisingly, the present isocyanate bonded wood composite, and method, solve this persistent problem, and provide a wood composite having strong surfaces without the need for a post-press reinforcing treatment and provide good porosity with respect to the adhesive.
Previous investigators have searched for alternative binders for wood composites that overcome the above-described disadvantages exhibited by traditional phenol-formaldehyde and similar resins. For example, Sharp, in U.S. Pat. No. 3,440,189, discloses the use of an isocyanate and a resinous extract of pine wood that contains hydroxyl moieties, together with wood fragments and a basic catalyst, to provide a particle board that demonstrates a water resistance comparable to plywood. Gallagher, in U.S. Pat. No. 4,100,138, discloses an isocyanate and polyether polyol binder as a replacement for the phenol-formaldehyde resin as the binder in a wood composite.
The isocyanate-based binders of both Sharp and Gallagher include intentionally added compounds having an active hydroxyl moiety which are present to react with the isocyanate. Further, neither Gallagher nor Sharp teaches or suggests that the particle board demonstrates improved porosity with respect to an adhesive and improved adhesive properties compared to a particle board including a traditional phenolformaldehyde resin binder.